Vehicle motion is highly dependent on the friction coefficient, i.e., the friction between the tires and the ground. Therefore an accurate estimate of this friction is valuable for many active vehicle safety functions, such as collision avoidance. For example, if low friction between the tires and the ground can be detected braking intervention can be performed earlier in a dangerous situation, and thus the risk of collision and serious injuries be reduced.
This is especially relevant for autonomous vehicles, as it is expected that autonomous driving will require estimates of the friction between the tires and the ground in order to adapt the vehicle speed (also referred to as velocity) automatically. This is relevant as e.g., driving too fast on a low friction surface, i.e., with low friction between the tires and the ground, could cause hazards such as skidding.
However, since it is assumed that the friction between the tires and the ground is difficult and costly to measure, the research society and automotive industry has been focusing on friction estimation, where “no-contact-to-ground” sensors are used, e.g., inertial measurement units, wheel-speed sensors, laser scanners, etc. A particular class of methods relies on knowledge of the physics of the tire where the relation between slip and tire force is known. A majority of the publications relating to such methods has hitherto shown that the tires must be significantly excited in order to enable such estimation, which means that the vehicle must e.g., either accelerate longitudinally or corner to exhibit large tire forces. An implication of that is consequently that during normal driving, where such excitation maneuvers occur stochastically at irregular intervals, it will be difficult or impossible to estimate the friction coefficient as sufficient excitation therefor is lacking.
One attempt at providing a method and apparatus for estimating road-to-tire friction between tires of a wheeled vehicle and a road surface, upon demand without disturbing a driver of the wheeled vehicle, is provided by document EP1481861 A1, which relates to a method for estimating the tire to road friction in order to enable an adaptation of a collision avoidance system to current road friction conditions. It suggests the use of a risk estimation module of a collision avoidance system to determine when to perform an automatic excitation of the tire-to-road contact surfaces, to enable estimation of the maximum available tire to road friction.
Automatic excitation, according to document EP1481861 A1, is performed when the collision risk estimated by the collision avoidance system exceeds a predetermined limit value. This limit value will be lower than the threshold value or values, which will actually trigger a collision avoidance intervention or collision warning by the collision avoidance system. It is further suggested to use the estimated friction to influence the decision mechanisms of the collision avoidance system, assuming that the estimated maximum friction level immediately in front of the vehicle will be the same or similar to the current conditions at the time of the excitation.
In accordance with document EP1481861 A1, a positive, driving torque is applied to both wheels on a first axle and an equal and opposite, negative, braking torque to at least one wheel on a second axle. Current values for vehicle speed, angular acceleration of the wheel on the second axle and the negative torque applied to said wheel are measured. A current friction coefficient is determined using a friction coefficient determining means. The positive torque may be applied by means of a propulsion unit connected to the first axle through a drivetrain for driving one or more wheels on the first axle, and the negative torque may be applied by actuating braking means for said at least one wheel on the second axle. It is said that in this way the driver of the vehicle will not experience a change in vehicle speed or an unexpected acceleration caused by the application of the brakes while the procedure for estimating the maximum available tire-to-road friction coefficient is performed.
However, although document EP1481861 A1 discloses that a friction coefficient determining means is used for determining a current friction coefficient from measured current values for vehicle speed, angular acceleration of the wheel on the second axle and the negative torque applied to said wheel, document EP1481861 A1 provides no further detail as to how this determination of the current friction coefficient is to be performed.
Vehicle speed, as required by document EP1481861 A1, is unfortunately very hard to estimate when all wheels are either braked or propelled since all wheels then will have high slip, and hence wheel speed sensors will not provide accurate values and thus give no clue as to the actual vehicle speed.
Thus, there is room for improving upon the previously suggested friction determination methodologies.